This invention relates to conditioning a gas stream, such as air, and especially to the dehumidification of a gas stream.
Conditioning of a gas stream, such as air, generally involves the removal or addition of moisture and the increase or decrease of temperature to make the gas stream suitable for its intended environment. For air conditioning in warm weather, this typically involves dehumidifying and cooling the air to comfortable levels.
Current dehumidification technology is based on the conventional, refrigerant vapor compression cycle (hereinafter referred to as DX technology) or on desiccant substrate capture technology (hereinafter referred to as DS technology). DX technology requires cooling humid supply air, such as the air within a room and/or outside air, to the water vapor condensation point, with external heat rejection on the compression side. This usually requires the supply air to be cooled below comfortable temperatures and, thereafter, either reheated or mixed with warmer air to raise its temperature to an acceptable level before directing it into the space being dehumidified. 20-35% of the energy expended in cooling the high humidity air is utilized to remove the latent heat from the air (the heat of condensation associated with water vapor condensation). Cooling and dehumidification of the air are thus coupled. That makes it impossible to independently control comfort parameters, making the DX cycle less efficient, from an overall system perspective, than a technology that would allow independent control of sensible and latent heat.
In applications where the outside air has both high humidity and temperature and the functional use of the interior space generates high water vapor levels (e.g. populated convention halls, exercise rooms, school buildings, etc.), it may not be possible for the DX technology to maintain the air introduced into the interior space at the correct humidity and temperature for maintaining comfort. The air delivered is cool but xe2x80x9cmuggyxe2x80x9d, since further cooling to remove additional water would result in the air being uncomfortably cool.
In stand-alone dehumidification using a conventional compression cycle, heat reject is in direct contact with the room air. As a consequence, the room air becomes more comfortable from a humidity side, but may be less comfortable (too warm) from a temperature parameter consideration. Again the comfort parameters are coupled.
DS systems are generally applied in central air, ducted systems. Water vapor is captured by capillary condensation on a solid phase substrate containing pores of the appropriate size (typically less than 100 Angstroms) to cause capillary condensation. The capture process is efficient and rapid. However, removal of the water vapor from the pores, wherein the intrinsic vapor pressure of the water is lowered in correspondence with the Kelvin equation, requires energy input. It also requires removing the substrate from the high humidity air stream and placing it in an exhaust, water reject stream, before adding the re-evaporation energy. Alternatively, the substrate may remain fixed and the treated air and exhaust streams flow directions interchanged as is done in a parallel bed, desiccant drier system.
In these DS systems, the re-evaporation energy is the latent heat of condensation plus the heat of adsorption of the water vapor in the substrate pore material. It is important to note that DS technology requires, in steady state operation, the addition of this energy at a rate equal to or greater than the latent heat of condensation of water in the desiccant substrate. That is, the water vapor reject power input must exceed the equivalent latent heat of condensation power. After water removal from the desiccant substrate the substrate must be re-cooled to the water capture temperature range of the substrate. As a consequence, some of the sensible heat of the subsequent cooling system (e.g. a DX cooling system) must be utilized in treating the DS substrate rather than for cooling the now dehumidified air.
The advantage of DS technology is that humidity levels in the outside air and/or recirculated air can be adjusted independently of the subsequent cooling step. The disadvantage is the requirement to move the substrate and treated air stream relative to each other for capture and rejection of the water vapor. This requires moving a large substrate through a sealed system, or, in a parallel bed DS system, requires complicated valving and valve cycling to move the humid air stream and an exhaust stream alternately across the desiccant beds. Again, application in typical stand alone, non-ducted room-type dehumidifiers is difficult if not impossible.
One object of the present invention is to provide an improved method and means for dehumidifying a gas stream.
Another object of the present invention is an efficient method and means for removing water from an air stream wherein the level of dehumidification is not interdependent with the temperature to which that stream may need to be ultimately cooled (for comfort or other purposes) before it is exhausted into the space being conditioned.
According to one embodiment of the method of the present invention, moisture is removed from a gas stream by bringing that stream into contact with the front surface of a hydrophilic capillary condenser layer that captures the water. An osmotic driving force, resulting from a water concentration gradient, transports the condensed water from the rear surface of the condensing layer through the thickness of an adjacent semi-permeable osmotic layer and into an osmotic fluid.
In apparatus used in the practice of the present invention, a porous wall is used to separate a moist gas stream from an osmotic fluid. The wall is comprised of a thin capillary condensing layer on the gas stream side with an osmotic layer (sometimes referred to as a semi-permeable membrane) disposed on the surface facing the osmotic fluid. In one embodiment the osmotic fluid is a solute dissolved in water, wherein the solute has a high ion (e.g., a salt) concentration; and the osmotic layer is a membrane permeable to water and not to ions in solution, such as a synthetic lipid bilayer. The choice of solute and any other additives making up the osmotic fluid will be determined by the transport properties through the membrane. In some cases, a biocidal component may be added in conjunction with a solute chosen for maximum flux through the membrane. The biocidal component is selected to prevent microbial growth or biofouling on surfaces which would naturally occur in an aqueous environment and eventually block the membrane or pores.
Examples of biocidal or bacteristatic additives that can exist in osmotic fluid as ionic species include silver and copper. In addition to these simple ionic antimicrobial agents, small concentration of larger molecules such as quaternary amines, or gluteraldehydes may be used. Gluteraldehyde is an example of a sterilant and disinfectant that is less corrosive than most other chemicals and does not damage plastics. Bleach (e.g. hypochlorous acid), for example, is antimicrobial but accelerates corrosion and would not be a preferred additive to the osmotic fluid.
Preferably the osmotic layer is in the form of a thin membrane adjacent to the surface of the capillary condenser layer. If the osmotic fluid is a solute dissolved in water, the membrane must have a material composition, thickness, pore size and porosity that must a) prevent the solute within the osmotic fluid from entering or blocking the pores of the membrane, and b) allow water to flow from the capillary condenser layer through the membrane and into the osmotic fluid as a result of a water concentration gradient level maintained during operation of the dehumidifier. In one embodiment the membrane is a lipid membrane layer, such as a synthetic lipid bilayer, disposed over the surface of the capillary layer. In another embodiment the membrane layer is made from collodion.
In another embodiment the osmotic fluid is one that is miscible with water at all concentrations, such as glycerol. Here the fluid can be maintained at a low water concentrations in order to maximize the osmotic flux. Typical membranes have permeabilities for glycerol which are about one thousand times less than for water. However, some reverse transport will likely occur.
The thickness of a typical synthetic lipid bilayer is about 5 nanometers. With layers that thin, water concentration gradients across a single bilayer or even a multilammelar layer can be large. This can provide a large driving force for water transport between the capillary layer and osmotic fluid. Water vapor condensing in the nanopores of the capillary layer will diffuse rapidly through the lipid layer and into the osmotic fluid. The high solute concentration of the osmotic fluid may be maintained in several different ways. For example, excess water may be evaporated or otherwise removed from the fluid; the solute may be replenished at appropriate times or intervals; and/or the fluid may be provided with excess solute (undissolved) that dissolves automatically when the concentration of water in the osmotic fluid exceeds the amount needed to have the water fully saturated by the solute. Other techniques or a combination of techniques may also be used to maintain a high solute concentration.
One of the primary benefits of the present invention is that the humidity of the incoming air may be controlled independently of the temperature. The water may be condensed out of the incoming humid gas stream onto the surface of the pores of the capillary condenser, taking advantage of the rapid and efficient water capture capability of capillary pore condensation technology and without the need to remove sensible heat from the air stream (i.e. the moisture may be removed from the gas stream at ambient temperatures). The water condensed in the capillary layer is caused to move through the osmotic layer and into the osmotic fluid by maintaining a water concentration gradient across the osmotic layer. The water concentration gradient across the osmotic layer is created and maintained by having a sufficiently low concentration of water (i.e. a high concentration of solute or miscible fluid) within the osmotic fluid.
The proper concentration of water in the osmotic fluid may be maintained over time by removing excess water from the osmotic fluid or by adding solute to the osmotic fluid. If it is assumed that the water vapor removed from the air is rejected to an exhaust area not in contact with the treated air, the now dehumidified gas stream may then be cooled to any desired temperature by appropriate means, such as by using a standard air conditioning cycle. The incoming air stream is thus made more comfortable by separately controlling both its humidity and temperature.
The present invention requires less energy to dehumidify a gas stream than do prior art methods. For example, re-evaporation power requirements for the present invention are lower than if the water were to be removed from the system by, for example, reheating a desiccant bed. This is because the osmotic fluid serves as a latent energy buffer for the captured water vapor (i.e. the heat of condensation released when water vapor condenses is buffered by the osmotic fluid). While it may be necessary or desirable, to use an energy source to assist in the removal (e.g. by separation or re-evaporation) of the excess water from the osmotic fluid, the process can be relatively simple and energy efficient compared, for example, to the analogous step of a DS cycle wherein a bed of desiccant is usually taken off line and heated.
The current system has the advantage of minimal moving parts and prolonged dehumidification capability. Even though the accumulated reject water must eventually be removed and energy must be expended, operation of the device may be continued for prolonged periods without such water removal. The reason this is permissible is that the water need not be separated or re-evaporated at the same rate or at the same time at which is it produced. If the water is directed outside, or where a lower humidity waste stream is present, or preferably where a source of waste heat is present (such as the condenser or compressor of an air conditioning system), the water may gradually evaporate with no additional work to be done by the system.
Capillary condensers that may be used in the present invention are well known in the art. The pore size and porosity of the capillary condenser layer are selected to assure that the water condenses onto the pore surfaces at a rate much faster than the rate at which it evaporates from the pores. The net amount of condensed water moves, by capillary action throughout the volume of the condenser to the interface of the condenser and the osmotic layer by capillary action and due to the hydrophilic nature of the condenser material. A thin capillary condenser layer may be supported on the surface of a thicker, larger pore condenser layer for structural integrity. Mounting the osmotic membrane on the back surface of the support can provide an effective spacer to keep capillary pressure from countering the osmotic forces. A micron thickness macroporous support between the capillary condenser and the membrane will accomplish this without reducing water flux significantly.
As mentioned above, an osmotic fluid that may be used in the method of the present invention is comprised of a solute dissolved in water. For example, the solute may be a salt. The solute and the osmotic layer are selected such that the size of the hydrated solute molecules are greater than the pore size of the osmotic layer in order to prevent the solute from flowing through the osmotic layer toward the capillary layer. The solute is selected such that molecules of solute do not cause blocking of the pores of the osmotic layer, which they would if they became lodged, to a significant extent, within the pores of the osmotic layer. To assure that the condensed water flows from the capillary condenser layer through the osmotic layer and into the osmotic fluid, a high concentration of solute is maintained in the osmotic fluid to maintain a high water concentration gradient across the osmotic layer.
In accordance with one embodiment of the dehumidification apparatus of the present invention, the combined condensing layer and osmotic layer define a porous wall within an enclosure. The wall, in combination with the enclosure, forms separate compartments on opposite sides of the wall. The compartment on the condenser layer side of the wall is the airflow compartment and the compartment on the osmotic layer side of the wall is the osmotic fluid compartment. High humidity air, which may be outside air and/or recirculated indoor air, is brought into and through the airflow compartment and passes over the condenser surface. Water vapor in the air condenses and travels to the interface between the condensing layer and osmotic layer through the capillary pores. The less humid air exits the airflow compartment and may then be cooled by separate air conditioning apparatus, if desired.
A solution of water and solute (osmotic fluid) is disposed within the osmotic fluid compartment. As a result of the water concentration gradient across the osmotic layer, the water within the capillary condenser layer travels from the interface between the condenser and osmotic layers, through the osmotic layer, and into the osmotic fluid. This results from an osmotic driving force created by the water concentration gradient across the osmotic layer. The dehumidification apparatus preferably includes means for regenerating the osmotic fluid to maintain a high concentration of solute in the osmotic fluid, and thus to maintain the high water concentration gradient across the osmotic layer during operation of the apparatus. For example, apparatus may be provided to evaporate, either continuously or as needed, excess water from the osmotic fluid.
The porous wall could also be the wall of a tube, with the above referred to compartments being the space within the tube (e.g. the osmotic fluid compartment) and the space surrounding the tube (e.g. the air flow compartment). The osmotic layer would be disposed on the capillary condenser surface on the inside of the tube.